Before you jump to conclusions, can you see a definite crack and break in the steel OR is the steel just worn away?

Mowing in sandy soil will erode a blade fairly quickly and even have a cracked appearance in the eroded area, but the area will have the rounded sandblasted look with smooth edges.

The remaining tabs have a clear CRACK along the bend, running from the end of the blade toward the center. This is not a "cracked appearance" as I can see through the crack. On the blades missing the tabs, there are jagged edges where the metal separated. All I'm suggesting is a quick look under your mower deck.

I am with baymee on this one. Inspecting the blades regularly should be part of normal maintenance, they do wear out and need to be replaced at regular intervals. I have seen so many worn down and split blades brought into the shop for sharpening.

Another reason to buy Gator Magnums, the extra thickness and strength is worth the extra few bucks. I am surprised you didn't feel the deck vibrating when it lost the pieces due to the blades being unbalanced.

By the way, my guess is you have a tractor manufactured by MTD.

How often do you sharpen the blades? Metal fatigue should be evident if the blades were sharpened more than every 3 or 4 years.

Another safety issue is people not walking the area to be mowed before mowing. I have three children, so it seems toys, rocks, etc. tend to find their way into the lawn and can become projectiles. This is especially bad when mowing next to a gravel road or driveway. This is why it is also important that others are not nearby when mowing (a commercial mower sent a rock through the back window of my Mom's car once as she drove by, pretty scary).